We have recently identified a novel family of developmentally controlled genes in the frog Xenopus laevis. We demonstrated for the first time that, using a conserved region of Drosophila development-controlling genes called the homeobox, it was possible to identify and isolate vertebrate genes that resemble the Drosophila genes in several respects. Since then, similar genes have been isolated by others from a number of vertebrates, including cDNA clones from human transformed cells. We have by now isolated one of the largest collections of homeobox-containing cDNA clones available in any vertebrate, which are expressed at the gastrula stage of development. Some of these genes are expressed maternally in the oocyte and will be the focus of our studies, because they could potentially be examples of the elusive "cytoplasmic determinants" thought to be laid down in egg cytoplasm for the control of the initial steps of embryonic differentiation. The objective of this proposal is to ascertain the developmental function of Xenopus homeobox-containing genes. A variety of techniques will be utilized towards this goal, and many of them take advantage of the ready availability of large amounts of Xenopus embryos during the earliest developmental stages, during which equivalent experiments with mammalian embryos would be much more difficult.